I ported my friends yz and what a diffrence,he races against the 4 pokes too,little compression,reeds.If you put holes in the box make sure you have a good filter,as it is with a closed box I clean his every week and its gross,The bike reacts well to a minor port job,If the cylinder is off might want to consider it.I have a crf250 4poke at my house rite now,my boys 125 eats it up